Never mind, found a decent enough explanation.. once again spoilers below.
The story goes that the colony ship (or resourcer ship, forget which) is hit by some virus or disease, which wipes out the intelligent aliens (the queen, commanders, whatever). The drones, being hardy, strong workers, appear to be able to fight off the infection, to a point. Hence, when they're discovered, they're unhealthy and starving. They're sick (from the disease, as well as probably hunger from having no guidance), but not wiped out.
Similarly, this explains why there was zero extra fuel available. Nobody was harvesting it or producing it.
Lastly, why the ship arrived on Earth: He speculated that because their technology is all organically-infused, the ship was smart enough that once it lost guidance, it self-navigated to a planet with the resources to sustain life, and Earth was the nearest hit.
I'll try to find the source for all this.